I want to get an extended battery, and found this HTC Rezound Extended Life Battery. It's really very cheap. Why? Did you get one before? How about yours?
When it comes to batteries, I suggest staying OEM or at least a reputable manufacturer like Laza or Mugen. There are lots of el-cheapo batteries out there that are exactly that, el-cheapo, and not worth the money you are paying. Even if you find a quality battery, be wary of the back door cover since in the Rezound this is where the antennas are, and many people have reported signal issues with cheap battery doors.
Here's why I think the extended battery makes the Rezound feel "fat". The Rezound by itself with the regular battery is thin enough. I have the silicone case on my Rezound. It makes it a little "fatter" but still comfortable and less likely to slip out of the hand. I wouldn't want to go without the silicon case in case of a drop. Hasn't happened yet, but it only takes one fall onto a hard surface to cause a lot of damage. I also keep the phone in a leather pouch in my pocket to prevent scratches. (I don't like screen protectors, they seem to make the touch screen less responsive.) I got the extended battery, and the silicone case designed for the Rezound with the extended battery. That is "fat". It won't fit in my leather pouch and feels like I'm holding a brick in my hand. Without the silicone case, the extended battery on the Rezound might be o.k. With the silicone case, it's just too bulky, IMHO.